but 放在开头
（1）(常用于否定句) 表示 而不;若不
But that I saw it,I wouldn't have believed it.
若不是亲眼目睹,我还不会相信呢.
（2）(用来引入新话题)那就
But now to the last question.
现在就来谈最后一个问题吧.